1. Field of the Invention
This invention relates generally to atomic-absorption spectrometry; and more particularly to a new spectrometer for obtaining multiple atomic-absorption measurements much more quickly, easily and accurately.
2. Prior Art
A commonly used instrumental analysis method for the elemental analysis of various materials for trace and minor elements is the atomic absorption method. In the most used manifestation the analytical instrument consists of a means for generating a vapor such as a nebulizer; a burner assembly to disassociate into free atoms the vapor delivered by the nebulizer: a source of monochromatic light such as a hollow cathode lamp which light is directed through the atomized vapor, and a device for isolating and measuring the monochromatic light after it has passed through the atomized vapor. The quantitative measurement of the elements present in the vapor and thus the liquid sample from which the vapor was derived is made by comparing the intensity of the monochromatic light characteristic of an element after absorption in the burner flame to the unabsorbed intensity of the light source.
The wavelength-region isolating device, called a monochromator, requires some mechanism for driving a diffraction grating (or other dispersive element). Some prior artisans have driven gratings with sine-bar devices, or direct motor-shaft-to-grating connections. Others have used nonlinear drums or cams for interconnecting grating arms with motors.
The sine-bar mechanism produces a drive whose angular rotation of the motor is substantially directly proportional to the wavelength passing through the monochromator. This drive requires a worm gear that is highly accurate throughout its length, and it requires a point of contact between the carriage that rides on the worm gear and the arm that slides along the arm. The worm gear and the sliding contact are both subject to wear.
Direct grating mount to a motor shaft does not introduce the wearing surfaces of the sine bar, and it allows rapid scanning of the wavelength. Connecting the motor shaft directly to the grating shaft produces only one mathematical relationship between the motor position and the wavelength passing through the monochromator: the relationship is nonlinear in wavelength and the inverse of wavelength (frequency or photon energy). A special high precision motor is required to achieve the necessary wavelength precision and accuracy, and the motor must be driven in such a manner that the nonlinearity is compensated for when wavelength scanning is recorded on a strip chart recorder for example.
Next we consider the nonlinear drum or cam (mounted on a motor shaft) that is in direct contact with the grating arm. In this case the wavelength range covered is limited by the extent of the motion of the arm as the cam rotates through one full revolution. A compromise is necessary between the range of the wavelength region covered and the accuracy and precision with which the wavelength can be set.
We shall only very briefly discuss prior-art devices for lamp positioning and alignment. They are generally rudimentary and unsatisfactory. Most of these devices have a lamp-carrying carousel. For accurate alignment each individual lamp must be manually adjusted on its mount.
Prior AA spectrometers are also limited by difficulty in obtaining optical absorption measurements in a favorable range. Precision and accuracy depend greatly on the range in which measurements are made.
It should be mentioned that some prior workers have used pulsed lamps. Pulsing of lamps allows signal handling on an a. c. basis, with various well-known advantages.
In atomic-absorption work it is necessary to collect some information as to the intensity of the light-source signal both in the presence of and in the absence of an absorber. This is usually done by atomizing distilled water, which is essentially nonabsorbing for the monochromatic light used to measure the element, and recording the signal; and then ceasing the introduction of distilled water into the flame and instead atomizing the liquid sample to be analyzed, and recording the signal. This is done rather frequently to compensate for changes in the light source intensity with time.
We will further mention that heretofore an important limitation of absorption methods is the limited dynamic range (of absorbance values and therefore concentrations) over which accurate measurements can be made.